1. Technical Field
The present inventions relate to circuits for transceivers and, more particularly, relate to compensation of transmit leakage into a receiver.
2. Description of the Related Art
Duplexers are typically used to couple transmitters and receivers to a shared antenna. When both a transmitter and a receiver operate simultaneously they allow full duplex operation. Typically in full duplex operation the transmitter and receiver operate in frequency bands that are different from one another. In a transceiver, however, even when the transmitter and the receiver operate in different frequency bands, leakage will bleed from the transmitter to the receiver. This occurs both within the duplexer itself and between components of the transmitter and the receiver.
Typically a receive signal is the weakest and the transmit signal is the strongest. The transmitted signal can be 90 dB higher than the received signal. A duplexer can have less than 50 dB suppression of the transmitted signal. The transmitted signal is the largest signal in the receiver. The receiver then has to extract the desired receive signal in the presence of this much larger transmit signal. Transmitter signal leakage into a receiver thus causes reduction in sensitivity of the receiver. Besides reducing transmit leakage into a receiver, it is desired to also improve sensitivity of the receiver.
Prior techniques for mitigating transmit leakage into a receiver used an additional receiver to cancel the transmit signal in the receive path. Two papers describing prior techniques for mitigating transmit leakage into a receiver were: “An integrated LMS Adaptive Filter of TX Leakage for CDMA Receiver Front Ends”, V. Aparin, G. J. Ballantyne and C. J. Persico, IEEE Journal of Solid State Circuits, Vol. 41, No. 5, May 2006 and “Integrated Blocker Filtering RF Front Ends”, A. Safarian, A. Shameli, A. Rofougaran, M. Rofougaran and F. D. Flaviis, IEEE Radio Frequency Integrated Circuits Symposium, 2007.